# Moving a sprite along a circle given by an implicit equation

I’m trying to create a hyperbolic version of Pong in a Poincaré disk, using C++ and SFML.

Here is my problem: when the ball rebounds on a paddle, I deduce two coordinates allowing me to recover the equation of the hyperbola (as a circle in the Poincaré disc), in implicit form:

$$x^2 + y^2 + ax + by + 1 = 0$$

So, I have my equation, and I can only move my sprite using its x and y Cartesian coordinates - I don’t know how to do it differently.

How can I move my sprite following this circle equation?

Moreover, I would like to represent the Poincaré metric, so the ball's velocity will also vary as a function of the metric and its position (slowing down on the screen as it gets closer to the edge of the disc where space is compressed)…

We can solve this by completing the square:

\begin{align} x^2 + y^2 + ax + by + 1 &= 0\\ \left(x^2 + ax \right) + \left(y^2 + by \right) + 1 &= 0\\ \left(x^2 + ax + \frac {a^2} 4\right) + \left(y^2 + by + \frac {b^2} 4\right) + 1 - \frac{a^2}4 - \frac{b^2} 4 &= 0\\ \left(x + \frac a 2 \right)^2 + \left(y + \frac b 2 \right)^2 &= \frac {a^2 + b^2} 4 - 1 \end{align}

This means the circle's center is at $$\\left(-\frac a 2, -\frac b 2\right)\$$ and has radius $$\r =\frac {\sqrt {a^2 + b^2 - 4}} 2\$$ (meaning we lack a real solution if $$\a^2 + b^2 < 4\$$)

You can compute an offset from the circle's center to your sprite's current coordinates to get its phase angle theta:

theta = atan2(sprite.y + b/2, sprite.x + a/2)


And then move your sprite along this circular arc by incrementing/decrementing theta (depending on whether you want to move counter-clockwise or clockwise).

newPosition.x = cos(theta) * radius - a/2
newPosition.y = sin(theta) * radius - b/2


I'll have to punt for now on the magnitude of the increment to theta that corresponds to a consistent speed under the Poincaré metric, but hopefully this gives you enough to get started. :)